1. Field of the Invention
The present invention relates to a projection apparatus with two panels, and more particularly, to a projection apparatus with two liquid crystal panels, which increases light emission efficiency and brightness by employing a color switching system.
2. Description of the Related Art
A projection apparatus with two liquid crystal panels can be manufactured to have the advantages of both a single-panel projection apparatus and a three-panel projection apparatus. The three-panel projection apparatus has an advantage of high light emission efficiency but has a disadvantage of a complex structure due to many optical elements. The single-panel projection apparatus has an advantage of a simple structure but has a disadvantage of low brightness. In particular, since the three-panel projection apparatus transmits more green light than blue and red light, a separate device to remove green light by 40% is additionally required in order to use the three-panel projection apparatus for a display such as a television.
Accordingly, there have been developed projection apparatuses with two liquid crystal panels, which provide the advantages of the single and three-panel apparatuses while also solving their problems. An example of such a projection apparatus with two liquid crystal panels is disclosed in U.S. Patent Publication No. 2002/0027619 A1. This projection system is shown in FIG. 1.
Referring to FIG. 1, P polarized white light 210 is polarized by an achromatic or neutral polarizer 215 and is then sequentially incident on a blue light modulator 220 and a green light modulator 230. The blue and green light modulators 220 and 230 actively modulate blue light and green light, respectively, in response to an analog voltage. Each of the modulators 220 and 230 can be an active stack or any other temporal modulator, which can shift the polarization of blue or green light in response to an applied voltage. Red light is passively changed by a first filter 240 to orthogonal polarization. A neutral analyzer 245 blocks light not changed by the modulators 220, 230.
Light output from the neutral analyzer 245 can be red+blue (i.e., magenta) light, red+green (i.e., yellow) light, red light, or white light. The polarization of the red light is rotated by a second filter 250. In the output light, mixtures with green or blue are separated into green and blue light having orthogonally polarized components by a polarizing beamsplitter 260, and the green and blue light illuminate a green/blue sequential panel 270. The red light is transmitted by the polarizing beamsplitter 260 and illuminates a red panel 280.
Panel pixels that are ON rotate the polarization by π/2, and this light passes through the polarizing beamsplitter 260 and is directed toward a projection lens 200. In order to avoid leakage of P polarized red light reflected from OFF panel pixels, a red/cyan polarizing filter 290 is further placed on an optical path between the polarizing beamsplitter 260 and the projection lens 200 to rotate the polarization of the red light to S polarization. A clean-up polarizer 295, which absorbs the leakage, is further provided on an optical path of the red/cyan polarizing filter 290, thereby increasing the contrast of an image displayed on a screen 205.
The two panel projection system disclosed in U.S. Patent Publication No. 2002/0027619 A1 uses the green/blue sequential panel 270 so that green and blue images cannot be simultaneously formed. Accordingly, the total amount of light is reduced by ⅔, decreasing the brightness of an image displayed on the screen 205.
A conventional screen to display an image is shown in FIG. 8A. However, the conventional screen is disadvantageous because it cannot combine yellow and magenta light having different optical paths.